1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a pneumatic radial tire provided with at least one layer of polyvinyl alcohol fiber cords as a belt reinforcing layer and having improved comfortability and durability.
2. Related Art Statement
The belt reinforcing layer in the pneumatic radial tire enhances the rigidity of tire tread portion to considerably improve the steering stability, wear resistance and low fuel consumption based on the rolling resistance in the tire.
Therefore, steel cords were mainly used as the conventional belt reinforcing layer, but organic fibers such as aramid fiber, rayon fiber, polyester fiber, polyvinyl alcohol fiber (PVA fiber) and the like were partly used.
In the case of steel cord, the aforementioned function is sufficiently achieved, but there are some problems in that the comfortability becomes poor due to high flexural rigidity of the tread portion, the rolling resistance is increased with the increase of tire weight as a whole to increase the fuel consumption being recently regarded as important, and water penetrates into the inside of the tire from a cut portion formed in the tread portion to cause corrosion of steel cord.
In order to overcome such problems of the steel cord, the organic fiber cord being naturally flexible and light and not causing corrosion is expected to be used as a belt reinforcing layer. However, rayon and polyester fiber cords conventionally used in the belt reinforcing layer are low in the modulus of elasticity as compared with the steel cord, so that they have a drawback that the hoop effect as the belt reinforcing layer itself is insufficient and the steering stability and wear resistance are poor.
Furthermore, when the aramid fiber cord having a high elasticity is used in the belt reinforcing layer, the hoop effect near so that of the steel cord is obtained. However, the flexibility of the organic fiber is not sufficiently developed because the dynamic modulus of the aramide fiber is too high, so that the comfortability is not improved. Furthermore, the fatigue resistance and the adhesion force, particularly adhesion force at high temperature, are low, so that the durability of the tire is not yet sufficient.
As to the PVA fiber, there has recently been found a method in which high strength and high modulus PVA fibers can relatively easily be supplied in a great amount (e.g. Japanese Patent laid open No. 60-126311 and No. 60-126312), whereby the use of PVA fiber as tire cord becomes possible, but there remains an anxiety in the safety of the tire in case of usual use method. That is, when the tenacity and elongation at break or end count of the cord are unsuitable in the tire using the high strength and high modulus PVA fiber as a cord for the belt, if the tire rides over stones on rough road or projections on road, partial breakage of the belt cords inside the tire is caused, so that such a fiber cord has a problem in tire safety. However, when the end count of the cords in the belt is increased for preventing the cord breakage, the separation failure is apt to be caused at the belt end.